Electric heating unit



May 22, 1934. A T 1,959,776

ELECTRIC HEATING UNIT Filed Dec. 3, 1929 V mm Inventor". Charles C.Abbott, by

His Attorheg.

- Patented May 22, 1934 1,959,776

UNITED STATES PATENT OFFICE ELECTRIC HEATING UNIT Charles 0. Abbott,Pittsfleld, Mass., assignor to General Electric Company, a corporationNew York Application December 3, 1929, Serial No. 411,372

" 4 Claims. (01. 201-67) My invention'relates to electric heating units,plates the provision of a method for making an more particularly toelectric heating units of the electric heater of the sheath core wiretype sheath wire type in which the resistance elewhereby it is possibleto form a continuous elecment is embedded in a compacted mass of intricheating unit having any desired length withsulating material enclosed bya metallic sheath, in very wide limits. 60

and has for its object the provision of an im- In carrying my inventioninto effect in one proved electric heating unit of this character, formthereof, I provide a heating element of the and a simple and eflicientmethod of making it. core wire type by winding a plurality of elec-Electric heating units of thesheath wire type trical resistanceconductors together in twisted are now in common use in electric heatersand form. During the swaging operation, the twist es the like. Electricheaters of this type may-have in the conductors will straighten out andthereeither a helical core of resistance wire or a by prevent an unduestrain in the conductors. straight core depending upon the service'to Ihave also provided suitable means for mainwhich the heater is to besubjected. The re-. taining the elongated core wire resistanceelesistance element or core generally is mounted ment central in themetallic sheath while the 70 centrally in the metallic sheath and iscompletesheath is being loaded with the insulating maly embedded in apowdered heat refractory interial and during the initial swagingoperation. sulating material, such as magnesium oxide, For this purpose,I indent or otherwise shape the compacted around the resistance elementin metallic sheath so as to provide flattened porsome manner as forinstance by reducing the tions at a plurality of points. Preferably, the75 diameter of the sheath. flattened portions will be positioned in thesheath Often-times, particularly in relatively long in pairs; theportions forming each pair being heating units, it is desirable to usethe straight positioned substantially at right angles to each corerather than the helical core as the resistance other. It will beunderstood that the sheath at element because of the specific resistancerethese flattened portions will be just large enough quired. However, ithas been very diflicult hereto receive the resistance element andthus,the tofore to make a relatively long continuous element will be securedin its central position heating unit of this type with a straight coreat a plurality of points spaced throughout the resistance elementbecause during the swaging length of the sheath. Afterthe sheath hasbeen 30 operation there was a considerable tendency for loaded with themagnesium oxide but before the straight core wire to break up intocomswaging,Icompact the insulating material which paratively shortlengths and thereby. open ciris located between each pair of flattenedporcuit the heating unit. tions so as to further insure the centrallocation This results from the fact that the metallic of the heatingelement. p

5 sheath stretches faster than the core wire re- I have further provideda simple and reliable sistance element and consequently, strains whichconnection between the terminal and the heatoften are sufliciently great.to cause rupture, are ing element whereby the unit may be swaged set upin thecore wire. Even though the core to any desired extent withoutinjury either to wire was not ruptured during the swaging opthe heatingelement or to the terminal. In car- 40 eration, it was found that in thefinished heater rying out this part of my invention, I have pro- 95 thecore conductor often had a very irregular vided a recess in the terminalintowhich an end cross section, certain portions of the wire beingportion of the twisted element is inserted. Then reduced nearly to thebreaking point thereby the diameter of the terminal is reduced so thatcausing bad spots. The heating element in servthe metal of the terminalis forced into the spaces ice frequently separated at these reducedporbetween the turns of the twisted element. This mo tions because ofthe alternate extreme heating terminal connection is claimedspecifically in my and cooling. copending application, Serial No.594,276, filed In an endeavor to overcome this difiiculty, it February20, 1932, which is a division of this has been customary to make theheating unit application.

in several portions and join these portions so as For a more completeunderstanding of my in- 105 to provide a. unit having the requisiteresistance vention reference should be had to the accomand length. Thispractice proved to be more or panying drawing in which Fig.1 is asectional less unsatisfactory because oftentimes the joints viewillustrating an electric heating unit connecessarily had to be locatedin very hot zones. structed in accordance with my invention before Inone-of its aspects, my invention contemthe indenting operation; Fig. 2is a fragmentary 1101 its terminal; Fig. 5 is an elevation of theterminal partly in section illustrating the terminal prepared forconnection with the heating element; Fig. 6 'is a sectional view showingthe assembled heating unit before the swaging operation; Fig. 7 isanelevation of one of the loading washers used in the insulating loadingoperation; and Fig. 8 is a partial elevation of the-finished electricheating unit constructed in accordance with my invention.

Referring to the drawing, in one form of my invention the elongatedheating unit 10 comprises a straight core wire resistance heatingelement 11 formed from a pair of twisted resistance conductors 11a, 11b,extending centrally of a metallic sheath 12. The resistance element ismade from a suitable high resistance material having considerableresiliency, such as nickel-chromium alloy. The heating elementll isembedded in a compacted mass of powdered heat refractory insulatingmaterial 13 such as magnesium oxide.

Metallic terminals 14 and 15 are provided for the heating element 11.Each terminal is provided with a stepped longitudinal recess 16 forreceiving a correspondingly stepped portion 1'7 of the twistedheatingelement. The stepped terminal portion of the heating element is formedby a reduced twisted portion 17a of the end portion of the heatingelement and by the end portion 17b of one of the conductors whichprojects beyond the twisted portion, preferably in the axis of theelement. It will be observed that when the terminal portion 1.7 isinserted in the stepped bore of the terminal, the reduced twistedportion 170. will be received in the larger terminal recess portion 16aand the projecting portion 17b will be received in the smaller recessportion 16b. Each terminal is compressed on its heating element terminalportion so that the metal of the terminal is forced into the spaces orgrooves between the turns of theltwisted core heating element. A very'good electrical and mechanical connection is thus provided between theterminals and the heating element. It will be observed that before aterminal could beremoved or pulled from the heating element, it would benecessary to strip the inner threads which are formed in my invention,the electrical resistance elements 11a and 11b are loosely twistedtogether with a relatively short pitch to form the resistance heatingelement 11. While I have. shown but two resistance strands wound abouteach other to form the twisted heating element, it is to be understoodthat several, as for instance three or four may be .used if desired.

The terminals 14 and 15 are then attached to their respective endportions of the heating element 11. The heating element 11 is preparedfor connection with the terminals 14 and 15 by providing the stepped endterminal portions 17. The projecting portion 17b may be formed bycutting oif or otherwise detaching a portion of one of the conductors(11a, 111)) so as to leave the single strand. After this, the twistedend portion within the sheath. It will be understood that the heatingelement will have a length sufficient to the larger terminal recessportion 1611. It will be understood that the projecting portion 17b willbe bent or twisted so as to lie in the axis of the heating element andfurther that the portion 16b of the terminal recess will have a diameterapproximately equal to that of the projecting strand 17b so that it willjust receive this portion. The terminal portion 17 is then inserted inthe recess 16 of the terminal, after which the terminal is swaged orotherwise compressed, preferably to a square cross-section, so that itsmetal is forced into the grooves or spaces between the twistedconductors of the terminal portions 17a of the heating element.

This terminal arrangement is a very important feature of my invention.It will be understood that if the twisted core wire heating element wereleft full size to the end rather-than having the stepped formation asshown, and the terminal were provided with a straight bore to receiveit, the terminal would be so weak at the end of the core wire that itwould practically draw in two at this point during the subsequentswaging of the assembled heating unit.

As has been pointed out, I provide the metallic sheath 12 with aplurality of flattened portions 12a positioned so as to maintain theheating element central during the insulating material loading operationand during the initial swaging operations. The sheath at these flattenedportions will be sufliciently wide to receive the heating element butordinarily will not be large enough for the terminals to pass through.Consequently it is generally necessary to assemble the heatingelementwith the sheath before the indenting operation.

The metallic sheath 12 is prepared for the.

permit theterminals to project fromthe ends of the sheath. The heatingelement may be supported and tensioned in the sheath by placing washers21in the counterbores 20 and which fit onthe terminal members so as tohold them centrally of the sheath. Suitable sleeves or collars 22 arethen provided on the terminal members and are secured by means of nuts23 which are tube and to prevent its kinking during the indentingoperation.

After the heating element has been assembled with the sheath as shown inFig. 1, the sheath is ready for the indenting operation. As shown (Figs.2 and 3), the indents are placed in the 14 metallic sheath so as toprovide the flattened portions 12a in pairs with the axes of theadjacent portions positioned substantially at right angles to eachother.It will be observed that by reason of this arrangement, the reducedportions of each pair taken together form a squareshaped retainingsection around the resistance element (Fig. 3). It will be understoodthat a number of these pairs will be spaced at intervals along thelength of the sheath.

It will be observed by reference to Fig, 3 that the passageway throughthe sheath at these flattened portions is sufficiently large to receivethe heating element 11 and by reason of the fact that the adjacentflattened portions are arranged substantially at right angles to eachother, the heating element will be secured centrally of the sheath at aplurality of points spaced at intervals throughout its length. .However,it is very important that the reduced sections of the sheath besufficiently large to prevent gripping of the resistance element at anypoint. Thus, generally there will be some clearance between the walls ofthe sheath and the element. It is preferable, therefore, to form eachpair of flattened portions in the sheath at substantially 45 to theadjacent pairs. In other words one pair of indents will be formed withtheir axes substantially at 90 to each other. Then the sheath will berotated aboutits longitudinal axis through 45 and'another set of indentswill be formed with their axes substantially, at 90 to each other. Ineffect,

therefore, the four indents of adjacent pairs taken together will forman octagonal shaped retaining section. This arrangement precludes anypossibility of the element moving from its central position beyond thelimits defined by the clearance between the resistance element and theside portions of the two square retaining sec-' tions formed by theadjacent pairs. It will be understood, of course, that if the axes ofthe flattened portions of each pair were not'thus rotated relatively tothe axes of the corresponding flattened portions of the adjacent pairs,the limits of the clearance would be defined by'the distance between thesurface of the resistance element and the corner portionsof the squareretaining sections.

It will be understood that the flattened portions will be spacedso thatthe heating element will be properly centered within the sheath but willnot be positioned so close to each other that the passageway through thesheath will be restricted. Moreover, it will be understood that theindents will be placed in the sheath not only to maintain the heatingelement 11 central but also to maintain the terminal members 14 and 15centrally of the sheath (Fig. 6).

removed and the interior of the sheath is thoroughly cleaned so that anyscale which might have become loosened by the indenting process isremoved. The interior may thus be cleaned by blowing dry air through thesheath.

The sheath is now ready for the loading operation. The sheath is closedat one end, for example at the left hand end, as viewed in Fig. 6, by apair of solid washers 21 which fit on the terminal 14 so as to hold theterminal centrally of the tube. These washers, as show'n,are received inthe left hand counterbore 20. The sleeve 22 is then positioned on theprojecting end portion of the terminal 14 and is secured by means of thenut 23. The terminal 15 is secured in a similar manner centrally of thesheath by means of a triangular shaped washer 26 which is positioned inthe counterbore 20 at the right hand end of the sheath. The resistanceconductor 11 is placed under a slight tension by turning the nuts 23inwardly on the terminals.

The sheath is now filled through the openings provided by the triangularwasher 26 with the heat refractory powdered insulating material 13. Toaccomplish this, the sheath is held in an upright position in a specialvibrating machine (not shown) with the end provided with the triangularwasher uppermost. As the sheath is filled with the insulating materialthrough the openings provided by the triangular washer, it is vibratedto shake the insulating material together and distribute it uniformlyaround the terminals and the resistance element; It will be understoodthat the powdered insulating material will readily pass through theflattened portions 12a to the lower portions of the upright sheath. Whenthe sheath has been filled, a solid washer 27 which is similar to thewashers 21 is inserted in place around the terminal 15 to close thesheath. The sleeve 22 and the nut 23 are then replaced and the core Wireresistance element 11 is placed under a slight strain by turning the nut23 inwardly on the terminal 15. The ends of the sheath are then turnedover so as to secure the washers in the ends of the sheath.

After the sheath has thus been loaded with the powdered insulatingmaterial, I compact the magnesium oxide in these portions of the sheathbetween the flattened portions 12a around the be accomplished bystriking the metallic sheath between each pair of flattened portionswith a ratherheavy impacting blow, (as by hitting the sheath with ashort split dye in a press. It has been found that this operation socompacts the magnesium oxide around the core wire that it will be heldperfectly central within the sheath during the swaging operations. Afterthe insulating material has thus been compacted, the nuts 23 and thesleeves 22 are again removed.

The entire unit is now passed through a swaging or rolling machine forthe purpose of compacting the insulating material within the sheath to ahard dense mass, the swaging operation reducing the diameter andincreasing the length of the unit. Several passes through the swagingmachine generally will remove the flattened portions of the sheathbulging the flattened sides outward somewhat, and it has been found thatthe sheath will assume a complete cylindrical shape throughout itslength. The sheath will lengthen somewhat faster than thecore wireduring the first two or three passes through the machine. In otherwords,the projecting terminals will be drawn within the sheath for a shortdistance. However, the insulating material will generally besufficiently compacted by the first several passes to secure theterminals against further inward movement. The twisted core wireresistanceduring the swaging operation will be lengthened so that theoriginal relatively short pitch of the wire will be greatly increased.Thus in one specific heating unit, the pitch which was approximatelybefore swaging was increased by the swaging operation to approximatelyH5. Moreover the terminals 14 and 15 are gradually r ed ucedin diameterby the swaging, the recessed ends contracting around having a finallength of approximately 232" which were swaged from an original lengthof 112". The diameter of this heating unit was reduced by swaging fromapproximately before swaging to .318" after swaging. It is to beunderstood, however, that units having much greater lengths may beconstructed by my method.

After the unit has been swaged to the desired extent, the Washers ateach end are cut off or otherwise removed and the ends of the unit aresquared off (Fig. 8). It will be understood that the terminals 14 and 15will be held firmly in stood that the ends of the units may then beprovided with any suitable terminal members such as those shown in thecompleted heater of Fig. 8.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The combination in an electric heating unit provided with a metallicsheath and powdered insulating material compacted in said sheath byreducing and elongating the sheath, of a resistance element embedded insaid insulating material having substantially the relatively lowresistance characteristics of a straight uncoiled resistance element,said resistance element being formed from a plurality of initiallystraight resistance conductors twisted together in electrical engagementwith each other throughoutsubstantially their entire lengths and beingthereby longitudinally yieldable without deterimental effects on saidresistance conductors when said sheath is reduced and elongated tocompact said insulating material. i

2. The method of making an electric heating unit having substantiallythe low resistance characteristics per unit length of a unit providedwith a straight resistance element which comprises twisting a pluralityof initially straight elongated resistance conductors together inelectrical engagement with each other substantially throughout theirentire lengths and with a relatively short pitch to form a resistanceelement, mounting said twisted conductors lengthwise in an elongatedmetallic sheath, filling said sheath with powdered insulating materialand compacting said insulating material by elongating and reducing saidsheath, the pitch of said twisted conductors increasing materiallyduring said,elonresistance element embedded in said insulating materialhaving substantially the relatively low resistance characteristics of astraight uncoiled resistance element, and terminal members secured tothe end portions of said element, said resistance element comprising aplurality of initially straight resistance conductors twisted togetherin electrical engagement with each other substantially throughout theirentire lengths and being thereby longitudinally yieldable to provide forthe reducing and elongation of said sheath sufiicient to compact saidinsulating material without the detrimental eifects on said resistanceconductors due to excessive strains such as would be produced in astraight uncoiled resistance element having the same relatively greatlength.

4. The method of forming an electric heating unit which compriseswinding a plurality of resistance elements together to form 'a twisterresistance heating element, mounting said resistance heating elementlengthwise within a metallic sheath, filling said sheath with powderedinsulating material,'swaging said sheath so as to compact saidinsulating material to a hard dense mass around said heating elementwith the end portions of said heating element released so as-topreventexcessive strains in said resistance element during said swagingprocess.

CHARLES C. ABBOTT.

